After spelunking into the dialer source code a bit, it seems the important stuff related to receiving and initiating calls are already handled by android's TelecomManager and the dialer only provides interface for users to interact with TelecomManager.

I haven't really dig into Messages source code these days, but a few years ago I was making an app that can send MMS messages and it was insane how much heavy lifting a messaging apps have to do in order to send MMS reliably on any carrier. Losing the default Message app might be a big blow considering so many 3rd party message app still can't do MMS right (at least the last time I checked). For example, people are still posting MMS issues on the github repo for SimpleMobileTools app you mentioned.